Integrated circuits are typically manufactured on a semiconductor substrate, such as a silicon wafer. The silicon wafer is a thin circular plate of silicon of 150 or 200 millimeters in diameter and approximately 25 mils thick. A single wafer has numerous integrated circuit devices that are imprinted on the wafer and comprise a lattice of devices. Each device consists of numerous layers of circuitry and a collection of bonding pads. The bonding pads are typically small pads of three mils square which serve as connections to the device pin leads.
Prior to the packaging of integrated circuit devices into carriers, the devices are often tested to avoid packaging bad devices. The testing process involves connecting a probe card to a tester. The probe card has a collection of electrical contacts or pins that stand in for the normal pins and wire leads of a packaged device. The wafer is then positioned so that the pins on the probe card make contact with a given device's bonding pads and the tester runs a battery of electrical tests on the device. A special machine, called a wafer prober, is used to position each device on the wafer with respect to the probe card. Each device on a wafer is tested in turn as a probe is driven by a stepper motor. High accuracy is required, because the bonding pads are small. If a probe card pin makes contact outside the pad area the device must be repositioned, slowing the testing process, and requiring operator intervention. Moreover, incorrect positioning can cause damage to the device since the probe card pins may break through the passivation layer of the device.
The primary purpose of wafer probing is to accurately position the wafer so that the device's bonding pads make sufficient electrical contact with a probe card's probe tips. Proper positioning requires that the prober accurately align a wafer to a specified axis relative to the motion axis of the prober motor. Typical methods of positioning a wafer in a tester involve taking an image of the die on a wafer using a video camera and determining a feature on the wafer which can be used to align an axis of the wafer with the motion axis of the probe motor.
Present known methods of aligning a wafer on a chuck often exhibit unreliable performance. Problems with these known methods may involve harsh lighting conditions which interfere with the operation of the camera or the presence of fine details in the image which produce incorrect reference points in low resolution vision processors.
It is therefore an intended advantage of the present invention to provide an accurate method of determining the rotation of an object in a vision processor.
It is a further intended advantage of the present invention to provide an angle correction for coarse alignment of semiconductor wafers in a probe tester through a method and apparatus which is insensitive to noise and environmental conditions.